Changing meter of score removes all line breaks
System configuration: MuseScore 3.5.0.28537 / Mac OS 10.13
Steps to replicate
New score with Meter at default 4/4
add a line bread at every fourth measure
Select all
See meter to 3/4
Surprising result: MuseScore removes all line breaks
This must be a bug, right?
Comments
This is by design.
If you have 2 4/4 measures on a system and change the time signature to 3/4 where do you think the system breaks should go? At the end of the new second measure? in the middle of the measure measure where the 9th beat of the system will be? Maybe 3 3/4 measures will fit nicely on the system. It doesn't make sense to keep the system breaks (which you call line breaks) or page breaks.
In reply to This is by design. If you… by mike320
Actually that was the way of the easiest implementation rather than a true necessity.
MuseScore could either assign existing breaks to the nearest end of measure, or at a very minimum keep the ones that are still at measure end.
Then the user can very easily select all breaks and delete them if necessary.
With the current behaviour the poor user is left with no other choice than to reintroduce all breaks manually.